A Novel Device For Applying Fluid Shear Stress On Cells And A Theoretical Model Of Cell Mechanics

Fluid shear stress(FSS)caused by fluid flow is an important mechanical factor around cells,and a large number of studies have demonstrated that FSS regulates the proliferation,apoptosis,differentiation and migration of bone cells or blood cells.Due to the complicated structure of biological tissue,it is very difficult to directly observe the relationship between mechanical stimulation and cell behavior.Therefore,in vitro loading techniques at cell level are the main methods of experimentally studying cell mechanics,but the present techniques have some specific disadvantages.In addition,the mechanical properties of cells vary along with the exertion of fluid flow so that the structure and function of cells will be regulated.Undoubtedly,establishing a theoretical model to precisely describe the mechanical properties of cells under fluid flow provides the basis for evaluating the functions of different types of cells or even for developing the rapid detection technologies.In this thesis,a novel device for applying fluid shear stress on cells was designed and a theoretical model of cell mechanics was established.Parallel-plate flow chamber is a traditional loading device for exerting fluid shear stress on cells,but it may cause cell contamination and is not conducive to long-term loading of cells.Inspired by cone-and-plate viscometer,we developed a novel device basing on commonly used six-well cell culture plate,which is able to exert FSS on cells cultured on the surface of plate.In addition,numerical simulations were done to study the effect of the physical parameters such as the angular velocity of cone plate and the viscosity coefficient of culture medium,as well as the geometrical parameters such as the cone angle,the shape of cone generatrix and the gap distance between the surface of culture plate and the vertex of cone,on the wall shear stress at the surface of culture plate.Finally the optimal values of the above parameters were obtained to produce uniform FSS of 1 Pa to 2 Pa in a large range.This device will be a easily-used in vitro technique for long-term mechanical stimulation on cells.Micropipette aspiration is usually adopted to measure the mechanical properties of cells.Our collaborators developed a modified micropipette so as to make cells entering into the pipe continuously.Basing on this newly developed technique,we established a theoretical model,through which the variation of cellular geometry is used to predict the mechanical properties of cells.The human lung cancer A549 cells were aspirated into a pipe by negative pressure suction,and the geometrical parameters of cells at different time points were recorded such as the radii and length of the cell front inside the pipe,the radius of the cell rear outside the pipe.Considering these geometrical parameters and the dynamic friction coefficient between cell and pipe wall,the equilibrium equations along the axial direction of the pipe was established,and furthermore the membrane tension of the cell was obtained.In addition,the plasma membrane tension consists of active tension and passive tension,in which the viscoelastic model is used to describe the passive tension.The experimental data were used to fit three unknown parameters,i.e.Young's modulus and relaxation time of cell membrane and dynamic friction coefficient.Compared with the classical Young-Laplace equation,the present model describes the dynamic process that cells enter into micropipette.The new technique of micropipette aspiration and theoretical model is able to accurately measure the mechanical properties of different types of cells,and one of its possible applications is to rapidly detect tumor cells with high throughput.